It is recognized that the gastrointestinal microbes of homeothermic animals serve several functions, such as digestion and development of the mucosal system, angiogenesis, and as a protection barrier against disease (Schiffrin, 2002; Stevens, 1998). The majority of these microbes are present in the digestive tract communities. They contribute to the harvest of dietary nutrients that would otherwise be inaccessible (Bached et al., 2004; Sonnenburg et al., 2005). The potential benefit of the microbes is to influence the absorption of some nutrients, such as lipids.

Thus, the establishment of a microbe is a key component in order to health by competitive mechanisms and immune system development and maturation. By facilitating the growth of protective bacteria that will compete against the opportunistic pathogens for food and adhesions sites to intestinal membrane, they can prevent the pathogens entering the body. An important aspect is the specificity of the host response, which depends on the bacteria species colonizing the digestive tract.

Therefore it is important to know the composition of this microbe in fish and the effects which different factors could have on it, especially in the case of fish culture with high economical interest such as E. coioides. The problem of this kind of studies is that culture techniques are laborious and less accuracy because all bacteria in the intestines only culturable bacteria that can be identified by this method. An alternative way to overcome these disadvantages in studying the composition of intestinal microbes is by molecular methods, polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE).

The intestinal microbe is an important component of mucosal barrier has resulted in the promotion of the use of beneficial probiotic (Gomes, 2008). Most of the studies with probiotic used the vegetative cells that possess the probiotic functions. The present study used endospores of B. subtilis, because endospore can be stored in long time, more stable in high temperature and pH, and resistant to feed pelletization.

The aim of research was determined growth performance, profile of the intestinal microbes and immune responses of orange spotted grouper E. coioides when fed on three different diets supplemented with different concentrations of probiotic B. subtilis.